In the case of the Hazel, it's an old kit that was mothballed for years and would perform terribly at retail.

As for the Woundwort, according to the leaks, Bandai wanted the marketing boost from exclusivity (Japanese consumers love limited edition products) and have better control of the numbers in circulation (avoid oversupply). I suspect they were afraid preorder numbers would not hold after release for reasons we won't know until we actually have the kit.